The ultrastructures of the cardiac muscle cells are similar to those in skeletal muscle. The sarcomeres contain similar contractile proteins: the thick and thin filaments form a hexagonal lattice, and the myosin heads (crossbridges) are arranged in an approximately helical array on the thick filament surface. Biochemically, the cardiac actomyosin ATP hydrolysis cycle follows the same pathway with similar kinetics as the skeletal ATPase cycle. While there exists a vast literature characterizing structural, mechanical and biochemical properties of the skeletal actomyosin interactions, comparatively little is known about cardiac muscle. Our ultimate goal is to apply the X-ray diffraction technique to study the mechanism of the Frank-Starling Law of the heart. X-ray diffraction is one of the few techniques that can simultaneously determine distances between the filaments and the fraction of cross-bridges bound to actin in a living muscle cell. One approach to gain insight into cardiac muscle contraction is to compare various characteristics of the crossbridges in the two types of muscle fibers. We were successful in obtaining low angle X-ray diffraction patterns from permeabilized cardiac muscle (the trabeculae), which were systematically compared with those from the skeletal muscle. Effects of temperature and ionic strength on the intensities of the myosin layer lines, the equatorial intensity ratio I1,1/I1,0 and the spacing of the filament lattice are similar in both muscles. While effects of temperature are similar, there appear to be some differences. Mainly, in the cardiac muscle, the myosin layer lines are weaker, the I1,1/I1,0 ratio tends to be higher and the lattice spacing D10, larger. These differences are consistent with the idea that under a wide range of conditions, a greater fraction of crossbridges is weakly bound to actin in the myocardium. The finding may lead to an explanation for the Frank-Starling Law.